Solid-oxide fuel cell assembly having simplified arrangement of current collectors

ABSTRACT

A solid-oxide fuel cell stack having a plurality of fuel cells connected in series and disposed as two stacks side-by-side in head-to-tail configuration, connected in series across one end and having cathode and anode current collectors mounted side-by-side on a stack base at the other end. Each current collector is a flat plate extending from the stack footprint. Surrounding the stacks is a cover-sealing flange so formed that when the cover is in place a thermal jacket is formed around the stacks. The current collectors are electrically insulated from the sealing flange and cover by a gasket, and extend outwards from the flange for electrical attachment to a load. This arrangement permits the stacks to be fully assembled and the stack cover secured and sealed in place without need for passing electrical leads through openings in the cover which must be subsequently sealed.

TECHNICAL FIELD

The present invention relates to systems employing a plurality ofhydrogen/oxygen fuel cells having a solid-oxide electrolytic layerseparating an anode layer from a cathode layer; more particularly, tosuch a fuel cell system wherein the plurality of cells is assembled inelectrical series in a stack having positive and negative terminalcurrent collectors for connecting to an electrical load; and mostparticularly to such a fuel cell system wherein the stack is thermallyjacketed by a cover and the current collectors extend from the cellsbeneath an edge of the cover.

BACKGROUND OF THE INVENTION

Fuel cells which generate electric current by the electrochemicalcombination of hydrogen and oxygen are well known. In one form of such afuel cell, an anodic layer and a cathodic layer are separated by anelectrolyte formed of a ceramic solid oxide. Such a fuel cell is knownin the art as a “solid oxide fuel cell” (SOFC). Hydrogen, either pure orreformed from hydrocarbons, is flowed along the outer surface of theanode and diffuses into the anode. Oxygen, typically from air, is flowedalong the outer surface of the cathode and diffuses into the cathode.Each O₂ molecule is split and reduced to two O⁻² anions catalytically bythe cathode. The oxygen anions transport through the electrolyte andcombine at the anode/electrolyte interface with four hydrogen ions toform two molecules of water. The anode and the cathode are connectedexternally through a load to complete the circuit whereby four electronsare transferred from the anode to the cathode. When hydrogen is derivedby “reforming” hydrocarbons such as gasoline in the presence of limitedoxygen, the “reformate” gas includes CO which is converted to CO₂ at theanode via an oxidation process similar to that performed on thehydrogen. Reformed gasoline is a commonly used fuel in automotive fuelcell applications.

A single cell is capable of generating a relatively small voltage andwattage, typically between about 0.5 volt and about 1.0 volt, dependingupon load, and less than about 2 watts per cm² of cell surface.Therefore, in practice it is known to stack together, in electricalseries, a plurality of cells. Because each anode and cathode must have afree space for passage of gas over its surface, the cells are separatedby perimeter spacers which are selectively vented to permit flow of gasto the anodes and cathodes as desired but which form seals on theiraxial surfaces to prevent gas leakage from the sides of the stack. Theperimeter spacers may include dielectric layers to insulate theinterconnects from each other. Adjacent cells are connected electricallyby “interconnect” elements in the stack, the outer surfaces of theanodes and cathodes being electrically connected to their respectiveinterconnects by electrical contacts disposed within the gas-flow space,typically by a metallic foam which is readily gas-permeable or byconductive filaments. The outermost, or end, interconnects of the stackdefine electric terminals, or “current collectors,” which may beconnected across a load.

A complete SOFC system typically includes auxiliary subsystems for,among other requirements, generating fuel by reforming hydrocarbons;tempering the reformate fuel and air entering the stack; providing airto the hydrocarbon reformer; providing air to the cathodes for reactionwith hydrogen in the fuel cell stack; providing air for cooling the fuelcell stack; providing combustion air to an afterburner for unspent fuelexiting the stack; and providing cooling air to the afterburner and thestack.

In an SOFC system, the fuel cell stacks typically are enclosed by ametal cover which forms a thermal barrier and cooling air manifoldaround the stack. The current collectors at opposite electrical ends ofthe stack must be accessible for connection to a load, and typicallyelectrical leads from the current collectors are passed through openingsin the cover. This approach presents problems in fabricating a fuel cellsystem, however, as the openings then must be reliably and permanentlysealed to contain the stack cooling gas. Further, connecting the powerleads to the stack typically must be done after the cover is in place.Both of these tasks can be complex and difficult and, if doneincorrectly or incompletely, can result in catastrophic failure of thefuel cell system.

What is needed is an arrangement of the stacks, current collectors,stack cover, and power leads which allows the covered stack to beassembled without requiring electrical leads to be extended through thecover.

It is a principal object of the present invention to simplify theconstruction of a solid-oxide fuel cell system, reduce its cost andsize, and improve its overall integrity as a stand-alone APU.

It is a further object of the invention to increase the reliability andsafety of operation of such a fuel cell system.

BRIEF DESCRIPTION OF THE INVENTION

Briefly described, in a solid-oxide fuel cell system, a fuel cell stackhaving a plurality of fuel cells connected in series preferably isprovided as two stacks disposed side-by-side in head-to-tailconfiguration, connected in series across one end and having cathode andanode current collectors mounted side-by-side on a stack base at theother end, the cathode current collector forming an end to one stack andthe anode current collector forming an end to the other stack. Eachcurrent collector is a flat plate larger than the footprint of itsrespective stack and extending from the stack footprint. Surrounding thestacks is a cover-sealing flange so formed that when the cover is inplace a thermal jacket is formed around the stacks. The currentcollectors are electrically insulated from the sealing flange and cover,and extend outwards from the flange for electrical attachment to a load.Preferably, a power lead is metallurgically secured to each currentcollector as by brazing or welding. This arrangement permits the stacksto be fully assembled and the stack cover secured and sealed in placewithout need for passing electrical leads through openings in the coverwhich must be subsequently sealed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the invention will be morefully understood and appreciated from the following description ofcertain exemplary embodiments of the invention taken together with theaccompanying drawings, in which:

FIG. 1 is a schematic cross-sectional view of a two-cell stack of solidoxide fuel cells;

FIG. 2 is a schematic mechanization diagram of an SOFC system inaccordance with the invention;

FIG. 3 is an isometric view from above of a two-stack fuel cellassembly, shown connected electrically in series between two currentcollectors;

FIG. 4 is an isometric view like that shown in FIG. 3, with a coverenclosing the stacks;

FIG. 5 is an elevational cross-sectional view taken along line 5-5 inFIG. 4;

FIG. 6 is an elevational cross-sectional view taken along line 6-6 inFIG. 4;

FIG. 7 is an equatorial cross-sectional view taken along line 7-7 inFIG. 4;

FIG. 8 is an isometric view from above, showing a fuel cell assemblycomprising the apparatus of FIG. 4 mounted on a manifold in accordancewith the invention, along with reforming, combusting, and heatexchanging apparatus for servicing the fuel cell stacks;

FIG. 9 is an isometric view from above, showing the fuel cell assemblyof FIG. 8 mounted in the lower element of a thermal enclosure;

FIG. 10 is an isometric view from above of an air supply assembly forcontrollably providing air to the fuel cell assembly shown in FIGS. 8and 9;

FIG. 11 is an exploded isometric view of a fuel cell system inaccordance with the invention, showing the air supply assembly of FIG.10 disposed in a structural enclosure, and showing the fuel cellassembly of FIG. 9 fully enclosed by both upper and lower elements of athermal enclosure;

FIG. 12 is an isometric view from above of a fully assembled fuel cellsystem in accordance with the invention;

FIG. 13 is an exploded isometric view from the front, showing amulti-element basal manifold in accordance with the invention fordistributing air and reformate fuel and exhaust products through andaround the fuel cell stacks, as shown in FIG. 8;

FIG. 14 is an isometric view from the rear, showing the manifold of FIG.13 partially assembled;

FIG. 15 is an isometric view from the rear, showing the manifold of FIG.13 further assembled;

FIG. 16 is a plan view of the lower level of chambers formed by thelower two elements shown in FIG. 13;

FIG. 17 is a plan view of the upper level of chambers formed by thethird and fourth elements shown in FIG. 13; and

FIG. 18 is a plan view of the uppermost element shown in FIG. 13,showing the mounting surface for the apparatus shown in FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a fuel cell stack 10 includes elements known in theart of solid-oxide fuel cell stacks comprising more than one fuel cell.The example shown includes two identical fuel cells 11, connected inseries, and is of a class of such fuel cells said to be“anode-supported” in that the anode is a structural element having theelectrolyte and cathode deposited upon it. Element thicknesses as shownare not to scale.

Each fuel cell 11 includes an electrolyte element 14 separating ananodic element 16 and a cathodic element 18. Each anode and cathode isin direct chemical contact with its respective surface of theelectrolyte, and each anode and cathode has a respective free surface20,22 forming one wall of a respective passageway 24,26 for flow of gasacross the surface. Anode 16 of one fuel cell 11 faces and iselectrically connected to an interconnect 28 by filaments 30 extendingacross but not blocking passageway 24. Similarly, cathode 18 of fuelcell 11 faces and is electrically connected to interconnect 28 byfilaments 30 extending across but not blocking passageway 26. Similarly,cathode 18 of a second fuel cell 11 faces and is electrically connectedto a cathodic current collector 32 by filaments 30 extending across butnot blocking passageway 26, and anode 16 of fuel cell 11 faces and iselectrically connected to an anodic current collector 34 by filaments 30extending across but not blocking passageway 24. Current collectors32,34 may be connected across a load 35 in order that the fuel cellstack 10 performs electrical work. Passageways 24 are formed by anodespacers 36 between the perimeter of anode 16 and either interconnect 28or anodic current collector 34. Passageways 26 are formed by cathodespacers 38 between the perimeter of electrolyte 14 and eitherinterconnect 28 or cathodic current collector 32. Anode spacer 36 andcathode spacer 38 are formed from sheet stock in such a way as to yieldthe desired height of the anode passageways 24 and cathode passageways26.

Preferably, the interconnect and the current collectors are formed of analloy, typically a “superalloy,” which is chemically and dimensionallystable at the elevated temperatures necessary for fuel cell operation,generally about 750° C. or higher, for example, Hastelloy, Haynes 230,or a stainless steel. The electrolyte is formed of a ceramic oxide andpreferably includes zirconia stabilized with yttrium oxide (yttria),known in the art as YSZ. The cathode is formed of, for example, porouslanthanum strontium manganate or lanthanum strontium iron, and the anodeis formed of, for example, a mixture of nickel and YSZ.

In operation (FIG. 1), reformate gas 21 is provided to passageways 24 ata first edge 25 of the anode free surface 20, flows parallel to thesurface of the anode across the anode in a first direction, and isremoved at a second and opposite edge 29 of anode surface 20. Hydrogenand CO diffuse into the anode to the interface with the electrolyte.Oxygen 31, typically in air, is provided to passageways 26 at a firstedge 39 of the cathode free surface 22, flows parallel to the surface ofthe cathode in a second direction which can be orthogonal to the firstdirection of the reformate (second direction shown in the same directionas the first for clarity in FIG. 1), and is removed at a second andopposite edge 43 of cathode surface 22. Molecular oxygen gas (O₂)diffuses into the cathode and is catalytically reduced to two O⁻² anionsby accepting four electrons from the cathode and the cathodic currentcollector 32 or the interconnect 28 via filaments 30. The electrolyteionically conducts or transports O⁻² anions to the anode electrolyteinnerface where they combine with four hydrogen atoms to form two watermolecules, giving up four electrons to the anode and the anodic currentcollector 34 or the interconnect 28 via filaments 30. Thus cells 11 areconnected in series electrically between the two current collectors, andthe total voltage and wattage between the current collectors is the sumof the voltage and wattage of the individual cells in a fuel cell stack.

Referring to FIG. 2, a schematic mechanization diagram of a solid-oxidefuel cell system 12 in accordance with the invention includes auxiliaryequipment and controls.

A conventional high speed inlet air pump 48 draws inlet air 50 throughan air filter 52, past a first MAF sensor 54, through a sonic silencer56, and through a cooling shroud 58 surrounding pump 48.

Air output 60 from pump 48, at a pressure sensed by pressure sensor 61,is first split into branched conduits between a feed 62 and a feed 72.Feed 62 goes as burner cooling air 64 to a tail gas afterburner 66having an igniter 67 via a second MAF sensor 68 and a burner cool aircontrol valve 70.

Feed 72 is further split into branched conduits between an anode airfeed 74 and a cathode air feed 75. Anode feed 74 goes to a hydrocarbonfuel vaporizer 76 via a third MAF sensor 78 and reformer air controlvalve 80. A portion of anode air feed 74 may be controllably diverted bycontrol valve 82 through the cool side 83 of reformate pre-heat heatexchanger 84, then recombined with the non-tempered portion such thatfeed 74 is tempered to a desired temperature on its way to vaporizer 76.Downstream of vaporizer 76 is a start-up combustor 77 having an igniter79. During start-up, when the reformer is cold or well below operatingtemperature, vaporized fuel is ignited in combustor 77 and the burnedgas is passed directly through the reformer to warm the plates thereinmore rapidly. Obviously, the start-up combustor is deactivated duringnormal operation of the system.

Cathode air feed 75 is controlled by cathode air control valve 86 andmay be controllably diverted by cathode air preheat bypass valve 88through the cool side 90 of cathode air pre-heat heat exchanger 92 onits way to stacks 44,46. After passing through the cathode sides of thecells in stacks 44,46, the partially spent, heated air 93 is fed toburner 66.

A hydrocarbon fuel feed pump 94 draws fuel from a storage tank 96 anddelivers the fuel via a pressure regulator 98 and filter 100 to a fuelinjector 102 which injects the fuel into vaporizer 76. The injected fuelis combined with air feed 74, vaporized, and fed to a reformer catalyst104 in main fuel reformer 106 which reforms the fuel to, principally,hydrogen and carbon monoxide. Reformate 108 from catalyst 104 is fed tothe anodes in stacks 44,46. Unconsumed fuel 110 from the anodes is fedto afterburner 66 where it is combined with air supplies 64 and 93 andis burned. When gases are below self ignition temperature, they areignited by igniter 67. The hot burner gases 112 are passed through acleanup catalyst 114 in main reformer 106. The effluent 115 fromcatalyst 114 is passed through the hot sides 116,118 of heat exchangers84,92, respectively, to heat the incoming cathode and anode air. Thepartially-cooled effluent 115 is fed to a manifold 120 surroundingstacks 44,46 from whence it is eventually exhausted 122.

Still referring to FIG. 2, a first check valve 150 and a first oxygengetter device 124 are provided in the conduit feeding reformate 108 tothe anodes (not visible) in stacks 44,46. A second check valve 152 andsecond oxygen getter device 126 are similarly provided in the conduitfeeding spent reformate 110 from the anodes to afterburner 66. Asdescribed above, during cool-down of the fuel cell stacks aftershut-down of the assembly, it is important to prevent migration ofoxygen into anode passages 24 wherein anode surface 20, comprisingmetallic nickel, would be subject to damaging oxidation. Each checkvalve includes a typical frusto-conical valve seat 154 receptive of avalve ball 156. Preferably, each valve 150,152 is oriented withinassembly 12 such that the ball is held in the seat by gravity whenreformate is flowed through the system in the forward direction. Thus,fuel flow opens the valve sufficiently for fuel to pass in the forwarddirection. When assembly 12 is shut down, each valve is closed bygravity. The valves may not be identical, as oxygen flows opposite tothe reformate in valve 152, but in the same direction as the reformatein valve 150; the so the balls and seats may require different weightsand/or sizes to function as intended. Each getter 124,126 includes apassageway 128 having an inlet 130 and an outlet 132 through whichreformate is passed during operation of the fuel cell assembly. Withinthe passageway is a readily-oxidized material 134 (oxygen-reducingmeans), for example, nickel metal foam, nickel wire or nickel mesh,which is capable of gettering oxygen by reaction therewith but whichdoes not present a significant obstruction to flow of reformate throughthe chamber. Nickel in the getters reacts with oxygen to produce nickeloxide, NiO, when the assembly is shut down, thus protecting thenickel-containing anodes from oxidation. When the assembly is turnedback on, reformate is again produced which, in passing through thegetters, reduces the NiO back to metallic nickel, allowing the gettersto be used repeatedly.

For clarity of presentation and to enhance the reader's understanding,the numbers of elements of the invention as presented further below aregrouped in century series depending upon the functional assembly inwhich the elements occur; therefore, elements recited above and shown inFIGS. 1 and 2 may have different numerical designators when shown anddiscussed below, e.g., stacks 44,46 become stacks 344,346.

Referring to FIGS. 3 through 7, in a fuel cell stack assembly 300 inaccordance with the invention, the cells 311 are arranged side-by-sideand may comprise a plurality of cells 311, respectively, such that eachof first stack 344 and second stack 346 is a stack of identical fuelcells 311. The plurality of cells is preferably about 30 in each of thetwo stacks. The cells 311 in stack 344 and stack 346 are connectedelectrically in series by interconnect 347, and the stacks are connectedin series with a substantially flat cathode current collector 332 and asubstantially flat anode current collector 334 on the bottom of thestacks. The current collectors are sized to have a “footprint” veryclose to the same dimension as a cover-sealing flange 340. The currentcollectors. preferably are adhesively sealed to a stack mounting plate338, and the stacks preferably are in turn adhesively sealed to thecurrent collectors. The sealing flange 340 for the cover 342 and top 343is then mounted and sealed to the current collector plates. A gasket 341between flange 340 and the current collectors is a dielectric so thatflange 340 does not cause a short between the current collectors. Powerleads 350,352 are attached to current collectors 332,334, respectively,through strong, reliable and highly conductive metallurgical bonds, suchas brazing. In this manner, the current collectors may pass under thecover mounting flange 340, with no additional sealing or power leadattachment required, and do not have to pass undesirably through thecover itself, as in some prior art stack assemblies. Passing leadsthrough the cover makes the assembly more complex and less reliable.

Referring to FIG. 8, a fuel cell assembly 400 in accordance with theinvention comprises stack assembly 300 operatively mounted on anintegrated fuel/air manifold assembly 500 which also supports first andsecond cathode air heat exchangers 600 and an integrated fuel reformerand waste energy recovery unit (“reforWER”) 1100. Assembly 400 receivesair from air supply system 900 (FIGS. 10-12) as described below andselectively preheats air going to the reformer. ReforWER 1100 reformshydrocarbon fuel, such as gasoline, into reformate fuel gas comprisingmostly hydrogen, carbon monoxide, and lower-molecular weighthydrocarbons, tempers the air and reformate entering the stacks,selectively burns fuel not consumed in the stacks, recovers heat energygenerated in various internal processes which would otherwise be wasted,and exhausts spent air and water, all in order to efficiently generateDC electric potential across power leads 350,352 (not visible in FIG.8).

Referring to FIGS. 9 and 11, enclosure of the fuel cell assemblycomprises two nested enclosures: a thermal enclosure 700 and astructural enclosure 800. Fuel cell assembly 400 is first disposed in a“clam-shell” type thermal enclosure 700, comprising a bottom portion 702and a top portion 704, which in turn is disposed in a structuralenclosure 800. The split line 706 between bottom portion 702 and topportion 704 is easily arranged such that all pipes, manifolds, shafts,power leads, etc., which need to pass between the “hot zone” 716 withinthe thermal enclosure and the “cool zone” 816 within the structuralenclosure, do so in the middle of split line 706. This provides for easyassembly of the hot components into the thermal enclosure. First, allhot zone components, included in assembly 400, are nestled into inbottom portion 702, which may be provided with a conforming well 708 forsecurely holding and cushioning assembly 400, as shown in FIG. 9. Themating surface 710 of bottom portion 702, along split line 706, isconfigured as required to accommodate the lower halves of the componentsextending through enclosure 700. Top portion 704 is configured tomatingly engage bottom portion 702. Top portion 704 is placed ontobottom portion 704 and may be sealed thereto along line 706 as desired.Thermal enclosure 700 may be formed of any suitable high-temperaturehigh-efficiency insulating material, as is known in the insulating art,and may be a composite including a light-weight metal case. The range ofsuitable insulating materials is expanded by removing the constraint ofoverall structural integrity afforded by providing a separate structuralenclosure 800.

Structural enclosure 800 preferably is fabricated from thicker metal,for example, to provide structural strength and a simple shape, such asa box with a removable lid, for ease of fabrication. Features such asbrackets, studs, electrical connectors, studs, weld-nuts, air intakeducts, and exhaust ducts, for example, may be part of the structuralenclosure for mounting internal components thereto and for connectingthe system to external structures. Features for vibration and shockisolation (not shown) may also be provided with the enclosure.

The air control assembly 900 is connected to elements of fuel cellassembly 400 projecting through split line 706, and assemblies 700,900are then installed within structural enclosure 800, as shown in FIG. 12,to form a fuel cell system 1000 in accordance with the invention.Preferably, control system 200 (shown schematically in FIG. 2 as powerconditioner 202, circuit protection I/O 204, drivers 206, and electroniccontrol unit 208, but not visible in FIG. 12) is also installed onboardthe system within cool zone 816 to minimize the number of discretesignals 210 which must be passed through enclosure 800 via connector820. Note also that high current capacity power leads also pass throughenclosure 800 via dual connectors 821.

Referring to FIGS. 13 through 18, an integrated fuel/air manifoldassembly 500 receives air via flexible bellows elements from air supplyassembly 900 and reformed fuel from reformer assembly 1100 and conveyshigh temperature air, exhaust, and hydrogen-rich reformate fuel to andfrom the core components of the system. Basal manifold assembly 500 isshown in FIG. 13 as comprising a three-dimensional assembly of threeperforated plates and two partitioned elements which are easily andinexpensively formed and which comprise a two-level network ofpassageways which allow for the mounting, close-coupling, andintegration of critical fuel cell system components, including heatexchangers, combustors, fuel reformers, solid-oxide fuel cell stacks,check valves, threaded inserts, and catalyzed and non-catalyzed filters.Of course, while a five-component manifold is shown for simplicity,within the scope of the invention any two of the perforated platesobviously may be incorporated into the partitioned elements, throughappropriate and obvious casting or moulding processes, such that themanifold comprises only three elements.

It should be noted that manifold 500 is actually two mirror imagemanifolds 500-1,500-2 sharing some common features, for example, cathodeair return from the stacks. Thus, reformate fuel flows from reforWERunit 1100 in two parallel streams to stacks 344 and 346 and is returnedto reforWER 1100 in two parallel streams. Likewise, cathode air flowfrom air supply assembly 900 is divided into two parallel streams andenters into each manifold 500-1,500-2 via mirror image couplings 902-1and 902-2 (FIGS. 8-10 and 13). Fuel cell assembly 400 thus is seen tohave its fuel cell stacks 344,346 connected in series electrically butserviced by gas flows in parallel.

For simplicity of presentation and discussion, except where functionsare unique, the following construction and function is directed tomanifold 500-1 but should be understood to be equally applicable tomirror-image manifold 500-2.

Bottom plate 502 is the base plate for the manifold and forms the bottomfor various chambers formed by combination of plate 502 with lowerpartitioned element 504, defining a lower distribution element 505, asshown in FIG. 16. Intermediate plate 506 completes the chambers inelement 504 and forms the bottom plate for upper partitioned element508, defining an upper distribution element 509. Top plate 510 completesthe chambers in element 508 and forms the mounting base for fuel cellassembly 300, heat exchangers 600, and reforWER unit 1100, as describedabove.

In operation, air enters a first bottom chamber 512 via coupling 902-1,flows upwards through slots 514-1,514-2,514-3 into heat exchanger 600-1,through the heat exchanger conventionally where the air is heated asdescribed below, downwards through slot 516-3 into a first upper chamber518, thence through opening 520 in plate 506 into a second lower chamber522. In chamber 518, the heated air is controllably mixed with cool airentering the chamber via bypass connection 904-1 from air supplyassembly 900. The tempered air flows upwards from chamber 522 throughopening 524 in plate 506 into a chamber 526 which defines a cathodesupply plenum for supplying reaction and cooling air upwards throughslotted openings 528 to the cathode air flow passages in stack 344.Spent air is returned from the cathodes via slotted openings 530 into acathode return plenum 532 and flows downwards through an opening 534 inplate 506 into a common cathode air return runner 536 leading into atail-gas combustor 1102 within reforWER 1100.

Hot reformate from reforWER 1100 enters manifold 500-1 via opening 538in top plate 510 and flows into chamber 540, thence downwards throughopening 542 into a feed runner 544, and upwards through opening 546 intoa chamber 548 defining an anode supply plenum for stack 344.

Preferably, opening 546 defines a seat for a valve having a ball 550(FIG. 14), preferably held in place by gravity, for allowing flow ofreformate during operation but preventing flow of oxygen into the anodeswhen the system is shut down. Further, preferably, chamber 544 and/or548 contains an oxygen-reactive material (not shown here but indicatedas 134 in FIG. 2), such as nickel wool, through which reformate mayeasily pass but which can scavenge any oxygen passing by ball 550 on itsway to the anodes.

Preferably, cathode supply chamber 522 and anode supply chamber 544 areconfigured to maximize the area of the common wall between them, suchthat chambers 522,544 define a co-flow heat exchanger which tends todecrease the temperature difference between the cathode supply air andthe anode supply reformate.

From chamber 548, reformate flows upwards through slots 552 into theanode flow passages in stack 344. Spent reformate (“tail gas”) flowsdownwards through slots 554 into an anode return plenum 556 and thencedownwards through opening 558 into a reformate return runner 560. Fromrunner 560, spent reformate flows upwards through opening 562 intoelongate chamber 564 common with manifold 500-2 and thence throughopenings 566 into the tail-gas combustor in reforWER 1100. Preferably,opening 562 is also formed as a check valve seat like opening 546 forreceiving a check ball 563 preferably held in place by gravity forpreventing reverse flow of oxygen into the anodes when the system isshut down. Further, preferably, chamber 556 and/or 560, like chamber548, contains an oxygen-reactive material (not shown here but indicatedas 134 in FIG. 2), such as nickel wool, through which the tail gas mayeasily pass but which can scavenge any oxygen passing by ball 563 on itsway to the anodes.

Burned tail gas from the combustor enters manifold 500-1 via slot 568-3and flows via slots 568-2,568-1 into bottom chamber 570 and thencethrough opening 572 into chamber 574 which acts as a supply plenum forcathode air heat exchanger 600-1. Burned tail gas flows upward fromchamber 574 through openings 576 and through heat exchanger 600-1, thusheating incoming cathode air, returning through openings 578 intochamber 580 and thence via openings 582 into a tempering jacket space354 (FIG. 7) surrounding stack 344 between the fuel cells 311 and cover342. The stack is thus tempered by the exhaust gas. The burned tail gasreturns from jacket 354 via openings 584 into an exhaust plenumcomprising openings 586-3,586-2,586-1 which is vented to the atmosphereby exhaust pipe 588 and pipe flange 590.

An SOFC system 1000 in accordance with the invention is especiallyuseful as an auxiliary power unit (APU) for vehicles 136 (FIG. 12) onwhich the APU may be mounted, such as cars and trucks, boats and ships,and airplanes, wherein motive power is supplied by a conventional engineand the auxiliary electrical power needs are met by an SOFC system.

An SOFC assembly in accordance with the invention is also useful as astationary power plant such as, for example, in a household or forcommercial usage.

While the invention has been described by reference to various specificembodiments, it should be understood that numerous changes may be madewithin the spirit and scope of the inventive concepts described.Accordingly, it is intended that the invention not be limited to thedescribed embodiments, but will have full scope defined by the languageof the following claims.

What is claimed is:
 1. A fuel cell system for generating electric powerby combination of oxygen with hydrogen-containing fuel, comprising: a) aplurality of individual fuel cells organized into at least one fuel cellstack including a plurality of cathodes and anodes; b) cover seal meansdisposed around said stack; and c) a substantially flat anode currentcollector plate and a substantially flat cathode current collector platedisposed side-by-side at an end of said stack and extending beyond saidcover seal means, said anode current collector plate and said cathodecurrent collector plate having a first footprint, said cover seal meanshaving a second footprint, wherein said first footprint is larger thansaid second footprint.
 2. A fuel cell system in accordance with claim 1wherein said fuel cells are solid-oxide fuel cells.
 3. A fuel cellsystem in accordance with claim 1 wherein said stack is organized as twoside-by-side stacks in head-to-tail relationship.
 4. A fuel cell systemin accordance with claim 1 further comprising a cover for enclosing saidstack, said cover being sealable against said cover seal means to form athermal jacket around said stack.
 5. A fuel cell system in accordancewith claim 1 further comprising means for electrically insulating saidseal means and cover from said current collectors.
 6. A fuel cell systemin accordance with claim 1 further comprising a stack base for receivingsaid stack and current collectors.
 7. A fuel cell system in accordancewith claim 1 wherein said system is mounted on a vehicle.
 8. A fuel cellsystem in accordance with claim 7 wherein said vehicle is selected fromthe group consisting of car, truck, boat, and airplane.
 9. A fuel cellsystem in accordance with claim 8 wherein said system is an auxiliarypower unit for said vehicle.
 10. An automotive vehicle, comprising afuel cell system for generating auxiliary power for said vehicle, saidsystem including a plurality of individual fuel cells organized into atleast one fuel cell stack including a plurality of cathodes and anodes,cover seal means disposed around said stack, and a substantially flatanode current collector plate and a substantially flat cathode currentcollector plate disposed side-by-side at an end of said stack andextending beyond said cover seal means, said anode current collectorplate and said cathode current collector plate having a first footprint,said cover seal means having a second footprint, wherein said firstfootprint is larger than said second footprint.
 11. A fuel cell systemfor generating electric power by combination of oxygen withhydrogen-containing fuel, comprising: a) a base; b) a substantially flatanode current collector plate and a substantially flat cathode currentcollector plate disposed side-by-side on said base, said anode currentcollector plate and said cathode current collector plate having a firstfootprint; c) a plurality of individual fuel cells including a pluralityof cathodes and anodes, said fuel cells organized as two side-by-sidestacks in head-to-tail relationship, one of said stacks being disposedon said anode current collector plate, and another of said stacks beingdisposed on said cathode current collector plate; and d) a coverdisposed around said plurality of fuel cells and mounted to said anodecurrent collector plate and said cathode current collector plate, saidcover having a second footprint, wherein said anode current collectorplate and said cathode current collector plate are positioned betweensaid cover and said base, and wherein said first footprint is largerthan said second footprint.
 12. A fuel cell system in accordance withclaim 11 wherein said cover includes a flange, wherein said flangedefines said second footprint.
 13. A fuel cell system in accordance withclaim 12, further comprising a gasket positioned between said flange andsaid anode and cathode current collector plates to electrically insulatesaid cover from said current collector plates.
 14. A fuel cell system inaccordance with claim 11 further comprising a top plate disposed on saidcover, wherein said top plate, said cover, and said base enclose saidfuel cell stacks to form a thermal jacket around said fuel cell stacks.15. A fuel cell system in accordance with claim 11 wherein said fuelcells are solid-oxide fuel cells.
 16. A fuel cell system for generatingelectric power by combination of oxygen with hydrogen-containing fuel,comprising: a) a base; b) an anode current collector plate and a cathodecurrent collector plate disposed side-by-side on said base; c) aplurality of individual fuel cells including a plurality of cathodes andanodes, said fuel cells organized as two side-by-side stacks inhead-to-tail relationship, one of said stacks being disposed on saidanode current collector plate, and another of said stacks being disposedon said cathode current collector plate; and d) a cover disposed aroundsaid plurality of fuel cells, said cover defining a split line with saidbase, wherein said anode current collector plate and said cathodecurrent collector plate are positioned between said cover and said base,and wherein said anode current collector plate and said cathode currentcollector plate extend beyond said split line.
 17. A fuel cell system inaccordance with claim 16 wherein said cover includes a flange thatdefines said split line with said base.
 18. A fuel cell system inaccordance with claim 17, further comprising a gasket positioned betweensaid flange and said anode and cathode current collector plates toelectrically insulate said cover from said current collector plates. 19.A fuel cell system in accordance with claim 16 further comprising a topplate disposed on said cover, wherein said top plate, said cover, andsaid base enclose said fuel cell stacks to form a thermal jacket aroundsaid fuel cell stacks.
 20. A fuel cell system in accordance with claim16 wherein said fuel cells are solid-oxide fuel cells.